JP4768605B2 - Method of manufacturing a rotating optical fiber having low polarization mode dispersion - Google Patents

Description

  The present invention relates to a method of manufacturing an optical fiber in which a fiber is drawn from a melt end of a preform, and then a portion of the fiber is twisted about a longitudinal axis by applying a torque to the fiber to impart rotation to the fiber. The present invention relates more particularly to this type of optical fiber having low polarization mode dispersion.

Light traveling in the optical fiber has two polarization modes. For optical fibers where the geometry and both internal and applied stresses are completely circularly symmetric, operation in the wavelength or wavelength range considered “single mode” inherently supports two orthogonal polarization modes. Here, these two polarization modes are degenerate, propagate at the same group velocity, and have no time delay after traveling the same distance through the fiber. In practice, however, optical fibers are not completely circularly symmetric. For example, two modes of degeneracy break due to imperfections such as geometric and morphological deformation and stress asymmetry. For example, see Non-Patent Document 1. As a result, the two polarization modes propagate with different propagation constants β ₁ and β ₂ . The difference in propagation constant is called birefringence δβ, and the magnitude of birefringence is given by the difference in propagation constants of the two orthogonal modes:

Due to birefringence, the polarization state of light propagating in the fiber periodically changes along the longitudinal direction of the fiber. The distance required for the polarized light to return to its original state is the fiber beat length, which is inversely proportional to the birefringence of the fiber. In particular, the beat length L _B is given by:

  Therefore, a fiber with a large birefringence has a shorter beat length and vice versa. Commercially available fibers exhibit various beat lengths because the geometric and stress asymmetry of such fibers varies along the length of the fiber and varies between different fibers. Typical beat lengths observed in practice range from as short as 2-3 millimeters (high birefringence fiber) to as long as 10-100 meters (low birefringence fiber).

  In addition to the periodically changing polarization state of light traveling through the fiber, the presence of birefringence indicates that the two polarization modes travel at different group velocities, and that the difference increases as birefringence increases. means. The time delay difference between the two polarization modes is called polarization mode dispersion or PMD. PMD causes signal distortion, and therefore PMD is very detrimental in high bit rate systems and analog communication systems. For a uniform linear birefringent fiber that is not perturbed, ie, externally unperturbed, the fiber PMD increases linearly as the fiber length increases. However, as the length increases, random mode coupling is inevitably introduced into the fiber due to externally applied perturbations, and therefore the increase in PMD along the fiber is statistically the fiber length. Is proportional to the square root of.

  Known methods to combat PMD intentionally rotate the warm fiber as it is drawn from the preform, and thus mechanical rotation “solidifies” in the fiber as it cools. This rotation of the birefringence axis in the fiber results in continuous mode coupling between orthogonal polarization modes of the signal being carried, thereby preventing the accumulation of significant phase shifts between the two modes. As a result, the PMD of the fiber is significantly reduced.

  The method described in the opening paragraph is known from US Pat. No. 6,057,056, in which the drawn fiber is passed through a roller whose tilt axis is tilted and the pulley is moved around an axis perpendicular to the spin axis. Can be moved. The swaying motion of the roller causes the fiber to twist along a substantial portion of its length. In particular, the portions of the warm fibers twisted in this way are then given a permanent twist (rotation) when their constituent materials cool down.

The cited literature specifies that the rotation applied to the fiber ideally has a non-constant spatial frequency. This can be achieved by tilting the pulley back and forth in an aperiodic manner. In this way, the method described above aims to achieve a PMD of less than 0.5 ps / km ^1/2 .
US Pat. No. 6,324,872 Rashleigh, SC, Journal of Lightwave Technology, LT-1: 312-331, 1983

  However, the known methods of rotating an optical fiber to reduce PMD have several drawbacks. For example, the quality of optical fibers produced today is gradually improved. Conversely, even unrotated fiber now has the ability to exhibit a PMD of less than 0.1 ps / km. Unfortunately, prior art spinning methods are not entirely successful in reducing this already low level PMD present in some of the single mode fibers produced today to even lower levels.

The present invention is, for example, 0.05 ps / miles than ^1/2, more preferably 0.03 ps / miles than ^1/2, even more preferably 0.02 ps / miles ^1/2, and most preferably 0.01 ps / miles ^It relates to a new and convenient method for producing optical fibers that can be used to produce fibers having a low PMD of less than ^1/2 . More specifically, the method of the present invention provides a conventional optical fiber preform that heats at least a portion of the preform to a conventional drawing temperature and is heated in such a manner that rotation is applied to the fiber. From each step of drawing an optical fiber. In other words, torque is applied to the fiber to twist the fiber about the longitudinal axis, resulting in twisting deformation of the fiber material in the hot zone. Rotation is “applied” to the fiber when the fiber material in the hot zone is twisted, and the twist solidifies in the fiber, thus making the fiber a permanent “rotation”, ie, a permanent twist. Indicates.

Applicants have alternated between a torsional period or rotational repetition distance, i.e., the direction of rotation, for a single mode optical fiber having a PMD of less than 0.1 ps / km ^1/2 in an unrotated state. It is desirable to rotate the fiber so that the distance required to do so is greater than 10 meters, more preferably greater than 20 meters, and most preferably greater than 30 meters. For example, for stepped single mode fibers (eg, fibers having a dispersion slope of 0.06 ps / nm ² / km and a dispersion of about 16 to 20 ps / nm / km at 1550 nm), such fibers are now commonly used It shows a beat length longer than about 30 meters.

  In certain preferred embodiments of the present invention, the peak fiber speed is greater than 1.5 revolutions per meter, more preferably about 1.2 to 4 revolutions per meter, although the invention is such The number of rotations of the fiber is not limited, and rotations faster than 4 rotations per meter and rotations less than 1.2 rotations per meter can also be used successfully.

The PMD level in an unrotated optical fiber is related to the beat length in that fiber, and generally a PMD of less than about 0.1 ps / km ^1/2 has a typical mode coupling length of about 10 meters. Corresponds to a beat length of about 5 meters. As a result, the method of the present invention is particularly applicable to single mode fibers having a beat length longer than 5 meters. Similarly, the method of the present invention is applicable to single mode fiber having a beat length longer than 10 meters, or even longer than 20 or 50 meters. Similarly, the method of the present invention is applicable to single mode fibers having PMD of less than 0.05 ps / km ^1/2 in the unrotated state.

A number of advantages over the prior art can be achieved using the method of the present invention. The present invention is particularly useful for providing low PMD in optical fibers that are single mode at wavelengths ranging from 1300 to 1625 nm, most preferably about 1550 nm. As a result, the present invention is embodied in a new type of low PMD single mode fiber and products (eg, fiber optic communication systems) comprising such a fiber. For example, low levels of PMD that have not been heard before can be achieved fairly consistently with long beat lengths (longer than 5 meters, more preferably longer than 10 meters) single mode fiber. For example, low in fiber rotation state as 0.05 ps / miles ^1/2, more preferably 0.03 ps / miles than ^1/2, even more preferably 0.02 ps / miles than ^1/2, and most preferably 0. A PMD of less than 1 ps / km ^1/2 can be achieved for the fiber using the method disclosed herein.

  Additional features and advantages of the invention will be set forth in the following detailed description, and in part will be apparent to those skilled in the art from the description, or may be set forth in the detailed description below, the claims, and the appended claims. It will be appreciated by practice of the invention described herein, including the drawings.

  The foregoing general description and the following detailed description present embodiments of the invention and are intended to provide an overview or arrangement for understanding the nature and characteristics of the claimed invention. Has been. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.

To account for PMD reduction, we define a parameter called PMD reduction factor, which is the ratio of PMD (τ ₀ ) of the same fiber in the unrotated state of PMD (τ) of the rotated fiber: PMDRF = τ / τ ₀ . For example, when PMDRF is 1.0, there is no improvement in the PMD of the fiber, while when PMDRF is 0.2, it means that PMD is improved by a factor of five.

Applicants have discovered that PMD reduction performance is strongly related to the fiber beat length and which type of fiber rotation technique is used. When using the long period rotation technique disclosed herein, the PMD of a fiber with a long beat length can be dramatically reduced. However, PMD reduction may not be effective when applying the same technique to short beat length fibers (eg, 1 meter beat length fiber). FIGS. 1 and 2 show the effect of the rotation period on two types of fiber, one having a beat length of about 1 meter and the other having a beat length of about 10 meters. In both FIGS. 1 and 2, the number of revolutions was 2.5 revolutions / m. A beat length of about 1 meter was common in what was considered to be a good quality optical fiber five years ago, but today's manufacturing process allows an optical fiber with a beat length of 10 meters or more. It has become possible to produce the product consistently. FIGS. 1 and 2 surprisingly show that the long period rotation technique does not achieve the same amount of PMD reduction for fibers with shorter beat lengths. Instead, long-period rotation techniques have a very beneficial effect on PMD of long beat length fibers, but those techniques are in practice for fibers with shorter beat lengths (eg, less than 5 meters). Not very effective. FIGS. 1 and 2 show the rotation period for two types of fiber with PMD reduction factors versus different beat lengths. 1 and 2, both α ₀ is the number of revolutions expressed in rev / m and Λ is the rotation period expressed in meters.

The result obtained using the long period sine wave rotation (speed) profile which takes the form of is shown. A sinusoidal rotation profile with a period of 20 meters is shown in FIG. As can be seen from FIG. 1, in a fiber with a short beat length, increasing the period of rotation may actually have a detrimental effect on the ability to rotate to reduce PMD in the fiber. On the other hand, as can be seen from FIG. 2, a fiber having a beat length of about 10 meters clearly shows improved PMD reduction at longer rotation periods. As a result, while the most common rotation techniques used in optical fiber manufacturing processes typically involve a small rotation period (eg, less than 5 meters), Applicants have identified PMDs for fibers with longer beat lengths. Use longer cycles to reduce effectively.

  The method of the present invention may be implemented using any device that can draw the fiber during the fiber drawing process and that can vary the frequency and / or amplitude of rotation. FIG. 4 shows such an apparatus that can perform the rotational function disclosed herein to impart the desired rotation to the optical fiber. Referring to FIG. 4, the furnace 20 is adapted to hold a preform 22 of the type commonly used in fiber optic drawing methods. The furnace 20 is attached to a frame 24 that defines a reference fixed frame for the drawing system. The frame 24 may be, for example, a building frame in which a fiber drawing operation is performed. A take-off or tension stand 26 having a pair of opposing draw rollers 28 is provided downstream of the furnace 20. The stand 26 includes conventional elements (not shown) such as an electromechanical drive system for rotating the roller 28 about an axis so as to draw the sandwiched fiber. A take-up reel 30 is also provided. The take-up reel is driven by conventional equipment (not shown) that rotates about an axis fixed relative to the frame 24 so as to wind the fiber from the stand 26 onto the reel. The furnace 20 is configured to hold at least a portion of the preform 22 in a soft, substantially molten state. The stand 26 is configured to pull the fiber 32 from the molten portion of the preform 22 so that the fiber passes along a substantially predetermined path.

  In the melting zone 34 near the upstream end of the path, the fiber is substantially melted. However, as the fiber moves downstream along the path, the fiber is cooled and solidified so that when the fiber reaches a point 36 significantly downstream from the furnace 20, the fiber is cooled to a substantially solid state. Is done. The area of the path that extends from the point 36 to the downstream end of the path is referred to herein as the “solid area” of the path. A cooling device 38 may be provided between the melting zone and the solid zone. The cooling device provides non-contact cooling, so it is desirable that no solids contact the surface of the fiber while cooling the fiber.

  A coating device 40 is also attached to the frame 24 in the solid area 36. The coating device is adapted to apply a polymer coating to the outside of the fiber. The coating device is also preferably a non-contact device. That is, the fiber passes through the coating device 40 without contacting or engaging any other solid. A suitable non-contact coating apparatus is disclosed, for example, in US Pat. No. 4,792,347. The above-described elements of the apparatus may be of conventional design commonly used in the field of optical fiber drawing. The apparatus may further comprise an additional guide roller (not shown) near the downstream end of the path 32 to divert the path from the fiber and hence straight, and further constrain the fiber in the path. Other conventional elements such as a quality inspection device may be provided.

  The rotation forming device comprises an adjustable carriage 46 slidably attached to the frame 24 for movement in a crossing path direction X transverse to the longitudinal direction of the path. A micrometer adjustment device 48 is provided for moving the carriage in the direction of the crossing path and once the carriage is adjusted to the desired position relative to the frame 24, to fix the carriage in place. By means of shafts 52 and bearings 54 so that the yoke 50 can pivot relative to the carriage and hence relative to the frame 24 about a pivot axis 56 extending in the direction of the crossing path and intersecting the path 32 at the intersection 58. A yoke 50 is attached to the carriage 46.

  The rotation imparting assembly 42 further includes a cylindrical first roller 60 attached to the yoke 50 for rotation about the first element axis 62. The roller 60 has a peripheral surface 64 that surrounds and is coaxial with the first element shaft 62. The frame of the motor 68 is attached to the carriage. The crank 66 is supported on the shaft of the motor 68 so that the motor 68 can rotate the crank 66 around an axis 70 parallel to the swing shaft 56. The connecting rod 72 has one end pivotally connected to the crank 66 away from the shaft 70 and an opposite end pivotally connected to the yoke 50 away from the swing shaft 56. Therefore, the rotation of the crank 66 around the crankshaft 70 is such that the roller shaft or first element shaft 62 is tilted to the position indicated by the wavy line 62 'in FIG. Alternatively, the yoke 50 can be swung around the rocking shaft 56 between the first element shaft 62 and the second terminal position inclined in the opposite direction to the position indicated by the wavy line 62 ″ in FIG. The end positions 62 ′ and 62 ″ are arranged with equal but opposite end tilt angles E1 and E2 from the nominal position 62 where the roller axis or first element axis is perpendicular to the longitudinal direction of the path 32. ing. However, at all positions of the roller including these end positions, the roller shaft 62 remains generally transverse to the longitudinal direction of the path. Each extreme tilt angle E is preferably between about 2 degrees and about 10 degrees from the nominal position. As discussed further below, the desired angle depends on the desired amount of rotation to be applied to the fiber. The angle E may be adjusted by adjusting the dimensions of the crank 66 and in particular the spacing between the pin connection of the connecting bar 72 and the shaft 70. The number of revolutions of the motor 68 determines the speed at which the yoke 50 and the first roller 60 swing between the two end positions. The motor 68 may be a stepper motor driven by a conventional type digital control system, a DC motor driven by an adjustable power source, a pneumatic motor driven by an adjustable gas supply, or any other conventional It may be a variable speed device such as a transmission motor. Alternatively, the motor 68 may be a constant speed device. Such a rotating device is further described in US Pat. No. 6,324,872, which is hereby incorporated by reference.

  Devices other than those shown in FIG. 4 may be used in the practice of the present invention. For example, U.S. Pat. Nos. 4,509,968, as well as U.S. Pat. Nos. 5,298,047, 5,897,680, and 5,704,960 which describe an apparatus for rotating a fiber about its axis as the fiber is formed. No. and U.S. Pat. No. 5,943,466. In general terms, a rotating device typically includes a fiber contact means that provides rotational force to the fiber, eg, a roller, and a drive means, eg, computer controlled, for moving the fiber contact means in a non-sinusoidal spatial pattern as a function of time. A drive motor and an associated mechanical connection for defining the movement of the fiber contact means are provided.

  Additional mechanisms for carrying out the method of the invention, such as for preform sine or non-sinusoidal rotation when preform rotation is used alone or in combination with the application of rotational force to the fiber. The mechanism will be apparent to those skilled in the art from this disclosure.

  The rotation period used here is preferably at least 10 meters, more preferably at least 20 meters, and even more preferably at least 50 meters, but Applicants have no upper limit on the period during which rotation is possible, and therefore The rotation period can be as long as 100 meters or longer. This is evident from FIG. 2, where the PMD reduction factor is plotted against the rotation period. Although the PMD reduction coefficient depends sensitively on the value of the rotation period, the general tendency is that the PMD reduction coefficient decreases as the rotation period increases. The maximum value of the PMD reduction coefficient gives the worst case judgment of PMD reduction performance. It can be seen from FIG. 2 that the PMD value due to the long-period rotation can be improved by about 10 times or more when the rotation period is longer than 20.0 meters even for a rotation speed of 2.5 rotations / m.

  PMD reduction using long period rotation is insensitive to typical process variations. Typical process related variations include rotational speed variations during the fiber drawing process, and some variation in fiber beat length along the fiber. Since the rotation of the fiber is performed by frictional thrust between the fiber being pulled and the moving wheel, the amount of rotation and the resulting number of rotations suffer from some inevitable variation. This variation can sometimes be as great as ± 0.5 revolutions / m. FIG. 5 shows the PMD reduction factor as a function of the rotation speed for a rotation period fixed at 20.0 m, and at longer rotation periods, the PMD reduction coefficient is 1.5 rotations, especially for various rotation speeds. It indicates that the rotation speed is higher than / m and low. For a rotation period of 20 meters, the PMD of the fiber can be improved more than 10 times at a rotational speed greater than 1.5 revolutions / m. Another process variation that can affect PMD reduction is fiber beat length variation. FIG. 6 shows PMD reduction as a function of beat length for different rotation periods at a fixed number of revolutions (2 revolutions / m), indicating that PMD reduction is dependent on beat length. For PMD reduction, beat length dependence is not a problem as long as the PMD reduction factor is sufficiently low. As a result, even for beat length fibers longer than 5 meters, excellent PMD reduction can be achieved using the long period rotation technique disclosed herein.

  The long cycle rotation profile used here to provide long cycle rotation can be of any periodic shape. The rotation profile used to provide long period rotation is preferably of a periodic shape where the fiber is first rotated clockwise and then counterclockwise. The periodic rotation profile is preferably a symmetrical rotation profile such that the magnitude of the maximum counterclockwise rotational speed is at least substantially equal to the magnitude of the fiber rotational speed when rotated clockwise. The shape of the periodic rotation profile can be any shape including, but not limited to, sine waves, triangles, trapezoids, or other periodic rotation functions with similar or longer rotation periods. Absent. The most preferable rotation profile used here is a sinusoidal rotation profile in which the magnitudes of the clockwise and counterclockwise rotation speeds are symmetric. However, the PMD reduction performance should be the same for other rotation profiles, so other rotation profiles can be used. FIG. 7 shows a typical form of a trapezoidal rotation profile. FIG. 8 shows the PMD reduction factor as a function of fiber beat length for several rotation periods at a fixed rotation speed of 3.0 rotations / m, with good PMD for rotation periods longer than 10 meters. It shows that reduced performance can be achieved.

  As described above, the rotation method disclosed herein is particularly beneficial for optical fibers having a long beat length. One preferred manufacturing method for manufacturing such fibers with long beat lengths is by the external (OVD) method. In the external method, a core layer is deposited on a cylindrical substrate. The central core region is generally deposited first on the mandrel or mandrel, and the mandrel or mandrel is removed after the soot has accumulated to a thickness sufficient to form the central core region. This central core region is then consolidated into a solid glass body and the central hole formed by removing the mandrel is closed. For example, the central hole is closed by drawing the consolidated soot co-appli foam to a smaller diameter core cane. The core cane of the central core region is then used as a substrate for additional core segment layers, if desired. In a preferred embodiment, an additional soot layer is deposited to form a segmented core profile having three or more core refractive index regions. In a preferred embodiment, the core refractive index profile has a central region having a refractive index Δ1 surrounded by a first annular region having a refractive index Δ2, and a second annular region having a refractive index Δ3 is a first annular region. It has at least three regions surrounding the region. Prior to the deposition of each soot region, the previous core region is preferably consolidated and redrawn to the core cane. Such a process, in which the various core regions are consolidated and redrawn to form a thinner core cane before additional soot regions are deposited, helps to form long beat length fibers. In addition, since the mandrel used in the initial soot deposition process used to form the central core region is relatively small, the central hole that must be closed in the central core region may be another deposition process (eg, MCVD). It is relatively smaller than that of the case. As a result, OVD, in particular, is a preferred technique for depositing soot necessary to form the optical fiber preform used herein. However, the present invention is of course not limited to such preforms, and other deposition methods such as, for example, MCVD, PCVD, and in particular VAD may be used.

  The invention is further illustrated by the following examples which are meant to be illustrative rather than limiting.

Example 1
A LEAF® optical fiber, which has a large effective area and is a non-zero dispersion shifted single mode optical fiber, is typically manufactured by first manufacturing an optical fiber preform and then heating the fiber to a temperature from which it can be drawn. Manufactured according to the fiber drawing process. “LEAF” fibers typically exhibit a certain beat length. For comparison purposes, the same fiber was drawn using both rotated and unrotated states, as well as various rotation periods. The birefringence characteristics of the unrotated fiber and the rotated fiber used to calculate the PMD reduction factor were considered to be ideally the same, and the beat length of these NZDSF fibers was longer than 20 m. A portion of the unrotated fiber was drawn and immediately after that a portion of the rotated fiber was drawn and this pattern was repeated with the same or different rotational conditions. “LEAF” optical fibers commercially available from Corning have an effective area of greater than 50 μm ² , more preferably greater than 60 μm ² , and most preferably greater than 70 μm ² . The fiber also has a zero dispersion wavelength of greater than about 1340 nm and less than about 1520 nm (more preferably between about 1400 and about 1500 nm). This optical fiber also exhibits a slope of less than 0.09 ps / km ² . A refractive index profile similar to that shown in FIG. 9 was used for this fiber. As can be seen from FIG. 9, the refractive index profile of the NZDSF fiber used a three segment core formed by doping the core region with various amounts of germania. In particular, the core has a first central core region 100 having Δ1, a second core region 102 surrounding the first core region 100 having Δ2, and a third surrounding the second core region 102 having Δ3. The core region 104 was used. However, the present invention is not limited to these types of refractive index profiles, and the three segments with Δ1>Δ3> Δ2 as in various refractive index profiles, in particular, the refractive index profile shown in FIG. A refractive index profile possessed may be used. In some cases, it may be desirable to dope the region 102 that is recessed on the graph with fluorine to provide a lower refractive index than the cladding, which is preferably undoped silica. By using different refractive index profiles as known in the art, a fiber having a dispersion slope of less than 0.07, even more preferably less than 0.06, in particular three of which Δ1>Δ3> Δ2 It can be easily achieved using the core structure of the segment. Using the long-period rotation technique disclosed herein, the PMD of such a fiber can be reduced, similar to the “LEAF” fiber shown in this example. A comparison of PMA of “LEAF” fiber in the unrotated state, the rotated state with a short rotation period of 1.5 meters, and the rotated state with a long rotation period according to the present invention is listed in Table 1 below. For a shorter period of rotation, the rotation speed is about 3.5 rotations / m, and for a longer period of rotation, the rotation speed is about 2.7 rotations / m. PMD values are expressed in units of ps / km ^1/2 . As can be seen from Table 1, using the long period rotation technique disclosed herein, the average PMD value is less than 0.01 ps / km ^1/2 (0.02 ps / km ¹ with the short period rotation technique for the same fiber). ^{/ 2} less compared to the), more preferably kept below 0.007ps / km ^1/2. Using the techniques disclosed herein, PMD values of less than 0.005 ps / km ^1/2 are achieved for such “LEAF” NZDSF fibers.

After drawing the fiber, several 1-kilometer samples each were wound on a 30 cm diameter measuring spool with zero winding tension. Large diameters and low winding tensions were chosen to reduce bending-induced fiber birefringence or PMD and external perturbations. Group delay difference (DGD) using a model HP8509 ellipsometer manufactured by Hewlett Packard, based on a mechanism called Jones matrix eigenanalysis to obtain DGD and PMD of the fiber under inspection Were further measured. Table 2 lists the PMD reduction factors for a standard short period rotation technique versus a long period rotation technique according to the present invention. As can be seen in both Tables 1 and 2, the long-period rotation technique used in accordance with the present invention achieved significantly better PMD reduction for this NZDSF fiber product. To the best of Applicant's knowledge, for such NZDSF fiber products using a segmented core refractive index profile with three segments Δ1, Δ2 and Δ3, where Δ1>Δ3> Δ2, the rotated fiber This is the first time that PMD has been reduced to less than 0.01 ps / km ^1/2 .

  It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. That is, the present invention is intended to embrace alterations and modifications of the present invention provided they come within the scope of the appended claims and their equivalents.

Graph showing the effect of different rotation periods on the PMD reduction factor in a fiber having a beat length of about 1 meter at a speed of 2.5 revolutions / m Graph showing the effect of different rotation periods on the PMD reduction factor in a fiber with a beat length of about 10 m at 2.5 rpm Graph showing rotation speed as a function of distance for a sinusoidal rotation profile with a rotation speed of 3.0 rotations / m and a rotation period of 20.0 m Schematic of a fiber rotator that can be used to carry out the method of the present invention. Graph showing PMD reduction factor as a function of rotation speed for a fixed rotation period of 20 m for a fiber having a beat length of 20.0 m Graph showing PMD reduction factor as a function of fiber beat length for 5 m, 10 m, 20 m and 50 m rotation periods at a fixed speed of 2.0 revolutions / m Graph showing rotation speed as a function of distance for a trapezoidal rotation profile with a rotation speed of 3.0 rotations / m and a rotation period of 20.0 m Graph showing PMD reduction factor as a function of fiber beat length for trapezoidal rotation profiles with 5 m, 10 m, 20 m, 50 m rotation periods, and 3.0 rpm Graph showing refractive index profile for NZDSF fiber

Explanation of symbols

20 Furnace 22 Preform 24 Frame 26 Winding stand 34 Melting zone 36 Solid zone 38 Cooling device 40 Coating device 50 Yoke 66 Crank 68 Motor

Claims (9)

5 m from have long beat length (L _B) a single-mode optical fiber having a birefringence (.delta..beta) represented by L B ₌ 2π / δβ corresponding to a permanent rotation exerted on the fiber Having a vertical axis
Rotation applied to the fiber has alternating with the counter-clockwise clockwise fiber rpm peak is the rotational speed or at least 1.2 revolutions per meter, repeat distance of the rotation of at least 20 meters It is the above length ,
A fiber characterized in that the repetition distance of the rotation is sufficient to give the fiber a polarization mode dispersion (PMD) of less than 0.05 ps / km ^1/2 . The fiber of claim 1 having a beat length greater than 20 meters. Claim 1, wherein the fiber, wherein the repeat distance of the rotation is the length of more than 30 meters. Claim 1, wherein the fiber, characterized in that rotation applied to the fiber is a sinusoidal rotation function. The fiber of claim 1, wherein the fiber has a beat length greater than 10 meters and the PMD produced by rotation applied to the fiber is less than 0.03 ps / km ^1/2 . The fiber of claim 1, wherein the fiber has a beat length greater than 10 meters and the PMD produced by rotation applied to the fiber is less than 0.01 ps / km ^1/2 . The fiber is
A central region (100) having a refractive index Δ1,
A first annular region (102) having a refractive index Δ2 surrounding the central region;
A second annular region (104) having a refractive index Δ3 surrounding the second annular region;
Including a core region having
2. The fiber according to claim 1, wherein [Delta] 1> [Delta] 3> [Delta] 2. In a method of manufacturing an optical fiber,
Heating at least a portion of the optical fiber preform, and drawing the optical fiber from the heated preform so that rotation is applied to the optical fiber by applying torque to the optical fiber. The rotation of the optical fiber about the longitudinal axis of the optical fiber such that rotation is applied to the optical fiber when the optical fiber is drawn from the preform by the torque;
Having
The optical fiber, a single mode optical fiber having a birefringence (.delta..beta) represented by L B ₌ 2π / δβ corresponding to length beat not longer than 5 meters (L _B),
Rotation applied to the optical fiber has alternating with the counter-clockwise clockwise fiber rpm peak is the rotational speed of more than at least 1.2 revolutions per meter, repeat distance of the rotation of at least 20 More than meters long ,
A method wherein the rotational repetition distance is sufficient to impart polarization mode dispersion (PMD) of less than 0.05 ps / km ^{1/2 to the} fiber.   Prior to the heating step, the preform core region surrounds a central region having a refractive index Δ1, a first annular region having a refractive index Δ2 surrounding the central region, and the first annular region. Forming the optical fiber preform by a process comprising depositing multiple layers of silica soot having a second annular region having a refractive index Δ3, wherein Δ1> Δ3> Δ2 The method of claim 8 further comprising the step of:

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